1. Field of the Invention
The present invention relates to a shunt protection module and method for series connected devices, and more particularly to a shunt protection module and method that have low power dissipation.
2. Description of Related Art
A conventional shunt protection apparatus is invented to ensure that series connected devices, such as multiple series connected LEDs, operate normally even if one of the series connected LEDs fails. However, designing a shunt protection apparatus that has low power dissipation is an important issue to be solved. U.S. Pat. No. 7,564,666 (related prior art hereinafter) disclosed a solution.
With reference to FIG. 7, the related prior art disclosed a shunt protection circuit. Each shunt protection circuit corresponds to and is connected to a LED and has a first terminal 51, a second terminal 52, a metal oxide semiconductor (MOS) gated silicon controlled rectifier (SCR) 53, two diodes 54, 55 and a resistor 56. The first terminal 51 is connected to an anode of the corresponding LED, and the second terminal 52 is connected to the cathode of the corresponding LED. A drain terminal of the MOS gated SCR 53 is connected to the first terminal 51, and a source terminal of the MOS gated SCR 53 is connected to the second terminal 52. The two diodes 54, 55 are back to back connected together. A cathode of one of the two diodes 54 is connected to the first terminal 51, and a cathode of the other diode 55 is connected to a gate terminal of the MOS gated SCR 53. The resistor 56 is connected between the gate terminal of the MOS gated SCR 53 and the second terminal 52.
The above-mentioned shunt protection circuit does not work when the corresponding LED operates normally so the shunt protection circuit conducts substantially zero current. When the corresponding LED fails and the failure of the corresponding LED results in an open circuit in the loop of the series connected LEDs, the MOS gated SCR 53 in the shunt protection circuit will turn on to shunt the current through the MOS gated SCR 53. Therefore, the series connected LEDs with one failed LED remain operating normally.
FIG. 8 shows another example disclosed in the related prior art. A zener diode 57 is applied to be the shunt protection circuit. A cathode of the zener diode 57 is connected to the first terminal 51, and an anode of the zener diode 57 is connected to the second terminal 52. The zener diode 57 conducts substantially zero current when the corresponding LED operates normally. When the corresponding LED fails and the failure of the corresponding LED results in an open circuit in the loop of the series connected LEDs, the zener diode 57 will conduct to shunt the current through the zener diode 57. Therefore, the series connected LEDs with one failed LED remain operating normally.
According to column 6 in the specification of the related prior art, a maximum power dissipation of the MOS gated SCR 53 or the zener diode 57 is a quarter of the power dissipation of the corresponding LED when the MOS gated SCR 53 turns on or the zener diode 57 conducts. For example, if a 350-milliampere (mA) current flows through the corresponding LED and a forward voltage of the LED is 3.5 volts when the corresponding LED operates normally, a power dissipation of the corresponding LED is about 1.2 watts so the maximum power dissipation of the MOS gated SCR 53 or the zener diode 57 is about 0.3 watts when the corresponding LED fails.
However, the power dissipation of the shunt protection apparatus disclosed in the related prior art cannot be lower than a quarter of the power dissipation of the corresponding LED. The reason is that an electrical potential difference between the gate and source terminals of the MOS gated SCR 53 has to be greater than a threshold voltage to switch the MOS gated SCR 53 to turn on. Based on the connections of the MOS gated SCR 53 shown in FIG. 7, however, an electrical potential difference between the drain and source terminals of the MOS gated SCR 53 will never be lower than the electrical potential difference between the gate and source terminals. Therefore, although the related prior art disclosed that the power dissipation of the MOS gated SCR 53 or the zener diode 57 preferably is no greater than about one tenth the power dissipation of the corresponding LED, it is practically difficult to be achieved by the invention disclosed in the related prior art.
To overcome the shortcomings, the present invention provides a shunt protection module and method that have low power dissipation to mitigate or obviate the aforementioned problems.